The 8 inch gun M1 was a 203 mm towed heavy gun developed in the United States. At 32,584 m (35,635 yds.), it had the longest range of any US Army field artillery weapon in World War II. It was also used in small numbers by the British Army.

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In 1919, the Westervelt Board described the ideal heavy gun for future development having a bore of 194mm to 8-inches, a projectile of about 200 lbs, and a range of 35,000 yards. More striking was that it be must be road transportable. At this time no other country had such a road transportable field gun.[1] Low priority design work occurred until 1924. Serious development began in June 1940 of an 8-inch (203 mm) gun that would have a range of 33,500 yards (30,600 m), a road speed of 25 mph (40 km/h), be transported in 2 loads weighing no more than 44,000 lb (20,000 kg) and be suitable for rail movement. The gun used the same projectile as the 8 inch coastal gun and the US Navy's 8 inch cruiser gun.[2] Using the same carriage as the 240 mm howitzer M1 eased development, but the gun was very troublesome and wasn't standardized until January 1942. The main problems were excessive bore wear and poor accuracy, but it was felt that nothing better could be produced in a timely manner. Thus it entered production at a low rate and in small numbers. The gun tube was produced by Watervliet Arsenal and the recoil system was produced by Hannifin Manufacturing. Watertown Arsenal, Bucyrus-Erie, and the S. Morgan Smith Company manufactured the carriage. Only 139 weapon systems were produced before production ceased in 1945.[3]

In the quest for greater tactical mobility, the Ordnance Department experimented with a self-propelled version. Like the 240 mm howitzer, it was mounted on a stretched Heavy Tank T26E3 chassis that had an extra bogie wheel per side as the 8in Gun Motor Carriage T93, but the war ended before they could be used and were later scrapped.

An 8-inch US Army field gun in action during the bombardment of Brest.

One of the requirements during development was that the gun be easy to emplace. Given the weight of the gun and carriage this was a challenge. Eventually, a pair of three-axle transport wagons were developed - one for the barrel/recoil mechanism and one for the carriage. These transport wagons were also used with the 240 mm howitzer. These were standardized as the M2 and M3. The six-wheeled wagon M2 carried the carriage and the M3 six wheeled transport carried the barrel and recoil system as shown below: This separate configuration required the use of the 20-ton M2 truck-mounted crane for set up. The crane included a clam-shell bucket that was transported on a trailer and used to dig the recoil pit for the gun. In spite of the weight and being transported in two pieces, the gun could be emplaced in two hours.[4]

The transport wagons could be towed by heavy trucks; but, had problems in soft ground. The M6 High Speed Tractor which was designated for towing the 8 Inch Gun and 240mm Howitzer was not yet in product; so, the Ordinance Depart improvised. Obsolete M3 Lee medium tanks and depot storage M10 tank destroyer were quickly modified and tested. These proved to be satisfactory and were adopted for use as the M33 and M35 respectively.[2]

The gun was assigned to non-divisional battalions that were under the control of GHQ Reserve. Eight battalions were organized trained and equipped. Each consisted of six guns organized into three batteries of two guns each. Five battalions were assigned to Europe (153rd, 243rd, 256th, 268th, and 575th) and three to the Pacific (570th, 573rd, and 780th).[5] They first saw action in Italy in April 1944 at the Anzio beachhead when two guns were assigned to the 698th Field Artillery Battalion.[6] Two guns also went to the Cassino front in Italy and were used in the counter-battery role against the German 170mm guns. They saw action in Europe where they were particularly effective against fortified targets and in counter-battery fire against German long-range artillery. At St. Malo France, two battalions of 8-inch guns participated in the siege. The 8-inch guns scored direct hits on the walls of the ancient citadel.[7] In the Siegfried Line Campaign, the 8-inch guns, adjusted by aerial observers, knocked out two bridges over the Roer River.[7] At the end of the war, the battalions were deactivated and the guns moved into storage; however, they were never again used in action and eventually disposed.

The gun fired separate loading ammunition with two increments. M9 Green Bag propellant was used for medium ranges and was preferred for improved accuracy and reduced barrel erosion. M10 White Bag was used for long and extreme range firing. Only two fuses were used: M51A3 point detonating (and delay), and M67A3 mechanical time. Range and muzzle velocity below are for maximum charge of M10 White Bag.